"Directional drilling" is a procedure employed in the earth drilling industry when it is necessary to change the direction of a borehole. In essence, directional drilling is achieved by inserting, at the downhole end of a drill string, a small section of pipe, called as a "sub", which has been "bent" such that the longitudinal axis at one end its ends is at a slight angle, referred to herein as the "offset angle" to the longitudinal axis at the other end its ends. Such a tool is called a "bent sub".
In practice, a vertical borehole is drilled to a predetermined depth. The drill string is then withdrawn and a bent sub having the desired offset angle is inserted between the end of the drill string and the downhole motor. The drill string is then tripped back into the borehole. Since the longitudinal axis of the drill bit will now be at an angle to the original borehole, the direction of the bore hole will be altered. The bent sub may be replaced any number of times in order to provide a borehole of the desired shape and configuration.
The angular offset of the bent sub is conventionally achieved by physically bending the sub or by otherwise modifying the longitudinal axis of the sub during the machining stage of its manufacture. Since the angular offset is very small, in the neighbourhood of one or two degrees, accurate bends are relatively difficult to achieve. Further, in order to provide for selectivity, it is necessary to provide a series of bent subs having offset angles ranging from about 1/4 of one degree in 1/4 degree increments to about 2 degrees.
While this system is workable, it has a number drawbacks. First, existing bent subs are difficult and, therefore, expensive to construct accurately. Second, this difficulty is multiplied by the number of different bent subs necessary to provide the selection offset angles required. Third, it is necessary to replace one bent sub with another when a change borehole direction is required. This is relatively time consuming.
A still further drawback of conventional bent sub systems relates to the location of the bent sub in the drill string. It will be understood that the more remote the location of the bent sub from the drill bit, the greater the interference between the drill string and the borehole when tripping the drill string into the borehole. Thus, it is viewed as desirable to position the bent sub as close to the drill bit as possible in order to reduce friction caused by the scraping of the drill string against the borehole wall. The optimum positioning of the bent sub would be between the power unit and bearing components of the downhole motor. However, for various reasons, this is not possible with conventional bent subs and, as a result, it has been inserted between the power unit and the downhole end of the drill string. This reduces the maximum allowable offset angle and thus increases the number of bent subs changes required to achieve the desired borehole configuration.